Conventionally, an apparatus has been known that determines whether an occupant is wearing the seat belt, and whether the seat belt is in an abnormal state (see, for example, Patent Document 1). This apparatus includes a buckle connection. detection mechanism that determines whether the buckle of the seat belt is released. If the buckle connection detection mechanism determines that the buckle is released to be in a non-connection state, this apparatus determines that the occupant does not wear the seat belt. On the other hand, if the buckle connection detection mechanism determines that the buckle is inserted to be in a connection state, this apparatus determines that the occupant is wearing the seat belt.
The apparatus also includes a motor to rewind the sear belt, and a calculation unit to determine whether the seat belt has been pulled out. The calculation unit determines that the seat belt has been pulled out if detecting that a counter electromotive force has been generated in the motor. Then, after the calculation unit has determined that the seat belt has been pulled out, if the apparatus determines that the seat belt is not worn after a predetermined time has passed, by using the buckle connection detection mechanism, the apparatus determines that the seat belt is in an abnormal state.